1. Field of the Invention
This invention relates generally to the field of magnetic recording, and more particularly, to a high efficiency multiple track longitudinal thin film tape head and a method for making the magnetic tape head.
2. Related Art
In the course of developing various systems for the storage of data, data processing systems have traditionally utilized magnetic tape as a data storage medium. In common tape drive systems the magnetic tape cartridge in which the magnetic media is enclosed is inserted into a tape transport system. The magnetic tape is then wound and rewound between a supply reel contained within the tape cartridge and a take-up reel in the tape transport system. The tape is transported along a tape path which brings the tape into contact with, or adjacent to, a magnetic tape head located along the tape path. Magnetic tape heads used in present-day tape drive systems are multi-track tape heads having separate read and write elements associated with each data track on the magnetic tape. This enables multiple track magnetic tape heads to read and write several streams of data (one per track) simultaneously. The magnetic tape is typically guided past the read/write head by air bearings which provide an interface of forced air with the magnetic tape to lower friction forces between the tape and bearing surface.
An example of a magnetic tape drive system which stores 18 tracks of data on a standard half-inch magnetic tape housed in the 3480-type cartridge is the StorageTek 4480 tape drive system, available from Storage Technology Corporation, Louisville, Colo., U.S.A. An example of a magnetic tape drive system which stores 36 tracks of data on the same half-inch magnetic tape is the StorageTek 4490 tape drive system, also manufactured by Storage Technology Corporation.
A typical tape head assembly for a magnetic tape drive comprises an approximately horseshoe-shaped core made from a magnetic material such as nickel zinc ferrite. A coil of wire wound around the core, referred to as the write coil, is used to induce a magnetic field within the core. The open end of the horseshoe forms what is referred to as a gap. Often times, the tape head manufacturing process leaves a second opening at the opposite end of the horseshoe. In such a configuration the first opening is referred to as the front gap since it is proximate to the magnetic tape media. The second opening which is further from the magnetic tape media is then referred to as the back gap.
For write operations, a time-varying electric current, referred to as a write current, is sent through the write coil. This write current produces a time-varying magnetic field in the core. If the core was a complete circle (for example, a toroid) the magnetic flux lines would travel in a circle along the core. However, the core does not completely close upon itself due to the front and back gaps. Rather, the magnetic flux lines bridge the front gap forming what is referred to as a gap field.
The magnetic tape is passed over the front gap at a predetermined distance such that the magnetic surface of the tape passes through the fringes of the gap field. As the write current changes, the gap field changes in intensity and direction. These temporal variations in the gap field result in a spatial pattern of magnetization on the magnetic tape. Thus, electronic data signals can be converted to magnetic signals and the data stored magnetically on the magnetic tape.
Recently, there has been a great demand for increasing the data throughput of magnetic tape transport systems used in conjunction with high-speed digital computers. In order to utilize the high-speed capabilities of these computers, it is necessary to increase the amount of data stored on a magnetic tape and to increase the speed at which the data is written to or retrieved from the magnetic tape media.
To increase the data storage capacity of the tape drive systems, the areal density of the magnetic tape media which stores the data must be increased. Areal density is defined as the number of units of data stored in a unit area of the tape. Areal density is determined by two components: the linear density and the track density. To increase the areal density of a magnetic tape media, one must increase either or both, the linear density and track density of the magnetic tape.
Track density is defined as the number of data tracks per unit width of magnetic tape. Two characteristics associated with track density are track width, defined as the actual width of an individual data track; and track pitch, defined as the distance from the center of one data track to the center of a neighboring data track. As magnetic tape head size decreases, the track pitch and track width of the magnetic tapes is decreased, thereby increasing track density.
An example of the increasing track density can be seen by comparing the StorageTek 18-track 4480 tape drive system with the StorageTek 4490 36-track tape drive system. As discussed above, both systems support the half-inch magnetic tape contained in the 3480-type cartridge. The magnetic tape used in the 4480 18-track tape drive system has a track pitch of approximately 630 .mu.m and a track width of approximately 540 .mu.m. The magnetic tape used in the 4490 36-track tape drive system has a track pitch of approximately 315 .mu.m and a track width of approximately 285 .mu.m. Thus, the track width of the 36-track system is approximately half the track width of the 18-track system. Also, the distance between data tracks in the 36-track system is approximately a third of the distance between the data tracks in the 18-track system.
Linear density is defined as the number of units of data stored per unit length of a magnetic tape data track. Present-day tape drive systems use chromium dioxide (CrO.sub.2) magnetic tapes. The maximum linear density of a magnetic tape is primarily a function of the composition of the magnetic tape media. To increase the linear density, magnetic recording devices are beginning to use high-coercivity tapes such as metal particle and metal evaporated tapes. High-coercivity magnetic tapes have enhanced high-frequency-response characteristics, enabling them to store data at linear densities which are substantially greater than the standard CrO.sub.2 magnetic tapes.
The coercivity of some metal particle tapes is substantially greater than the more common thick/thin CrO.sub.2 tapes. Thus, the gap field required to write to metal particle tapes is correspondingly greater than that to write to CrO.sub.2 tapes. This larger gap field is typically greater than that which can be generated using conventional ferrite heads. With conventional ferrite heads, the gap field strength is substantially proportional to the write current, but only up to a threshold level where the magnetic material on either side of the front gap (the pole tip) saturates. After this saturation point is reached, increasing the write current results in little or no increase in the gap field strength. This phenomenon is known as "pole-tip saturation." For example, a NiZn ferrite magnetic head begins to saturate when writing to tape media having coercivity of approximately 600 Oersteds. The MnZn ferrite magnetic head begins to saturate when writing to tape media having a coercivity of approximately 1,000 Oersteds.
One approach to writing to high coercivity multiple magnetic tapes without experiencing pole tip saturation is described in commonly owned U.S. utility patent application entitled, "Multi-Track Longitudinal, Metal-In-Gap Head," Ser. No. 08/094,322, herein incorporated by reference in its entirety. This type of magnetic tape head is capable of achieving the necessary write current to write to high coercivity tapes. However, as track density increases, the glassed closure portion of the Metal-in-Gap (MIG) magnetic tape head becomes fragile and fractures as the size of embedded ferrite poles decreases. Thus, the multiple track MIG magnetic tape head is only extendable to a maximum of approximately 300 tracks per inch (TPI).
Another drawback of the MIG magnetic tape head design is that an unintentional magnetic gap referred to as a "pseudo-gap," forms between the high saturation flux density material and the ferrite due to the different materials of each. As a result, the magnetic flux lines bridge this unintentional gap as well as this front gap, thereby deteriorating the recording performance of the magnetic tape head.
Another drawback of the MIG magnetic tape head design is that the track definition which can be achieved is limited by the tolerances associated with the ferrite sawing techniques which are used to form the substrate and closure sections.
Another magnetic head design directed to writing to high coercivity magnetic tapes is the full thin film head (TFH). Magnetic heads having thin film pole pieces were developed to increase the saturation moment of the pole pieces and to increase the operating efficiency of the transducers. However, there are a number of drawbacks to conventional thin film heads.
One drawback of the TFH design is that the winding patterns, which are deposited on the substrate surface, have to be planarized prior to depositing the top pole piece. This increases the fabrication costs and decreases the yield of multiple track thin film magnetic heads.
Another drawback to conventional TFHs is the resultant thick coil planarization stack requires a sloped region in the top pole in order to form the direct gap. This sloped region of the top pole is prone to saturation, thereby limiting the write current which may be achieved.
Another drawback of the thin film magnetic head is the inability to withstand the abrasion on the magnetic recording medium, especially in the contact recording situation. High density recording requires the minimization of spacing between the surface of the tape media and the functional recording gap between the pole pieces. With the soft magnetic material, thin film poles are susceptible to wear by the media. This results in the pole piece wearing down in a direction away from the tape media, thereby increasing the size of the recording gap. This prevents conventional thin film magnetic heads from maintaining the necessary flux density at the location of the tape to write to high coercivity magnetic tape media.
Another drawback of the of the thin film head design is that the yield on multi-track arrays is low due to the multitude of processing steps which are required. Typically, more than fifteen photolithographic steps are performed in manufacturing conventional TFHs. When manufacturing a multiple-track magnetic head, wherein a flaw in a single track results in the complete magnetic head being unusable, this number of lithographic steps results in the fabrication process having a low yield.
One thin film head which has experienced some of the above problems is described in U.S. Pat. No. 5,142,768 to Aboaf et al. (Aboaf). In Aboaf, a method for manufacturing magnetic tape heads is described which includes the deposition of a thin film of soft magnetic material onto the substrate. This requires the substrate to be planarized which decreases the yield as described above. In addition, the magnetic head design described in Aboaf experiences the wash out problems described above. To overcome this problem, a conductor is deposited on the ferrite and the write element is operated in the opposite direction.
Thus, what is needed is a longitudinal multiple track magnetic tape head capable of writing data to high coercivity magnetic tapes at increasingly higher track densities. The write element must achieve the necessary gap field strength without experiencing pole tip saturation. In addition, the write element must require few fabrication steps to achieve an acceptable yield of multiple track magnetic tape heads.